Gasification is a process that enables the production of a combustible or synthetic gas (e.g., H2, CO, CO2, CH4) from carbon-based feedstock, referred to as carbonaceous feedstock. The gas can be used to generate electricity or as a basic raw material to produce chemicals and liquid fuels. This process enables the production of a gas that can be used for generation of electricity or as primary building blocks for manufacturers of chemicals and transportation fuels.
In particular, the gas can be used for: the combustion in a boiler for the production of steam for internal processing and/or other external purposes; for the generation of electricity through a steam turbine; the combustion directly in a gas turbine or a gas engine for the production of electricity; fuel cells; the production of methanol and other liquid fuels; as a further feedstock for the production of chemicals such as plastics and fertilizers; the extraction of both hydrogen and carbon monoxide as discrete industrial fuel gases; and other industrial heat requirements as required.
Gasification is not an incineration or combustion process. Both incineration and combustion processes operate to thermally destroy the carbonaceous feedstock with excess oxygen to produce CO2, H2O, SO2, NO2 and heat. Incineration also produces bottom ash and fly ash, which must be collected, treated, and disposed as hazardous waste in most cases. In contrast, gasification processes operate in the absence of oxygen or with a limited amount of oxygen and produce a raw gas composition comprising H2, CO, H2S and NH2. After clean-up, the primary gasification products are H2 and CO.
In contrast to incineration, which works with excess air to fully convert the input material into energy and ash, gasification converts carbonaceous materials into energy-rich fuels by heating the carbonaceous feedstock under controlled conditions. Gasification processes deliberately limit the conversion so that combustion does not take place directly. Gasification processes operate at substoichiometric conditions with the oxygen supply controlled (generally 35 percent of the O2 theoretically required for complete combustion or less), enabling gasification to convert the carbonaceous feedstock into valuable intermediates that can be further processed for materials recycling or energy recovery. Some gasification processes also use indirect heating, avoiding combustion of the carbonaceous feedstock in the gasification reactor and avoiding the dilution of the product gas with nitrogen and excess CO2.
Generally, such a gasification process consists of feeding carbon-containing materials into a heated chamber (the gasification reactor) along with a controlled and limited amount of oxygen and steam. At the high operating temperature created by conditions in the gasification reactor, chemical bonds are broken by thermal energy and by partial oxidation, and inorganic mineral matter is fused or vitrified to form a molten glass-like substance called slag.
Apart from municipal solid waste, hazardous waste, etc., coal of varying grades can be used as the feedstock. This includes low grade, high sulfur coal, which is not suitable for use in coal-fired power generators due to the production of emissions having high sulfur content. Waste coal particles and silt that remain after coal has been mined, sorted and washed is also be useful for gasification. Coal can be gasified with oxygen and steam to produce so-called “synthesis gas” containing carbon monoxide, hydrogen, carbon dioxide, gaseous sulfur compounds and particulates. The gasification step is usually carried out at a temperature in the range of about 650° C. to 1200° C., either at atmospheric pressure or, more commonly, at a high pressure of from about 20 to about 100 atmospheres.
There are several different types of coal, each displaying different properties resulting from geological history. The degree of coal development is referred to as a coal's “rank.” Peat is the layer of vegetable material directly underlying the growing zone of a coal-forming environment. The vegetable material shows very little alternation and contains the roots of living plants. Lignite is geologically very young (less than 40,000 years). It can be soft, fibrous and contains large amounts of moisture (typically around 70%) and has a low energy content (8-10 MJ/kg). Black coal ranges from 65-105 million years old to up to 260 million years old. These are harder, shinier, less than 3% moisture and can have energy contents up to about 24-28 MJ/kg. Anthracite contains virtually no moisture and very low volatile content, so it burns with little or no smoke. It can have energy contents up to about 32 MJ/kg.
Because coal often contains sulfur compounds, attempts have been made to provide processes for the gasification of coal to produce a clean product fuel gas wherein the sulfur is removed from the product fuel gas prior to its use, e.g., in gas turbines to generate electricity. In addition, gases from the gasification zone may be purified to remove coal dust and fly ash and also many other impurities, e.g., vaporized ash, alkali, etc.
There are a number of patents relating to different technologies for the gasification of coal for the production of synthesis gases for use in various applications, including U.S. Pat. Nos. 4,141,694; 4,181,504; 4,208,191; 4,410,336; 4,472,172; 4,606,799; 5,331,906; 5,486,269, and 6,200,430.
Many different types of biomass are appropriate for use as feedstock in gasification processes for the production of synthesis gases. For example, biomass useful for gasification include pulp and paper waste, wood products such as shredded bark, wood chips or sawdust, sewage and sewage sludge, food waste, plant matter, rice straw, agricultural and animal waste, and cellulosic type industrial waste (e.g., construction wastes). In fact, biomass, as used in the present context, is defined to include any substances of biological origin that can be utilized as an energy source or industrial raw material. Since biomass is produced by solar energy, and by the action of air, water, soil, or similar natural substances, it can be produced infinitely, and therefore provides an unlimited source of carbon for use in gasification processes for the production of synthesis gases.
There are a number of patents relating to different technologies for the gasification of biomass for the production of synthesis gases for use in various applications, including U.S. Pat. Nos. 6,948,436, 6,987,792, 6,923,004, 6,991,769, 6,808,543, 6,830,597, 6,680,137, 6,685,754, 6,615,748, 6,032,456, 5,425,792, and 4,699,632.
Plasma torch technology has also been employed in coal and biomass gasification processes. A plasma arc torch is created by the electrical dissociation and ionization of a working gas to establish high temperatures at the plasma arc centerline. Commercially-available plasma torches can develop suitably high flame temperatures for sustained periods at the point of application and are available in sizes from about 100 kW to over 6 MW in output power.
Plasma is a high temperature luminous gas that is at least partially ionized, and is made up of gas atoms, gas ions, and electrons. Plasma can be produced with any gas in this manner. This gives excellent control over chemical reactions in the plasma as the gas might be neutral (for example, argon, helium, neon), reductive (for example, hydrogen, methane, ammonia, carbon monoxide), or oxidative (for example, oxygen, nitrogen, carbon dioxide). In the bulk phase, a plasma is electrically neutral. Thermal plasma can be created by passing a gas through an electric arc. The electric arc will rapidly heat the gas by resistive and radiative heating to a very high temperature within microseconds of passing through the arc. A typical plasma torch consists of an elongated tube through which the working gas is passed, with an electrode centered coaxially within the tube. In one type of such torch, a high direct current voltage is applied across the gap between the end of the center electrode as anode, and an external electrode as cathode. The current flowing through the gas in the gap between the anode and the cathode causes the formation of an arc of high temperature electromagnetic wave energy that is comprised of ionized gas molecules. Any gas or mixture of gases, including air, can be passed through the plasma torch.
The gaseous product of the gasification of coal and biomass is called “synthesis gas” (or syngas), and contains carbon monoxide, hydrogen, carbon dioxide, gaseous sulfur compounds and particulates. Gasification is usually carried out at a temperature in the range of about 650° C. to 1200° C., either at atmospheric pressure or, more commonly, at a high pressure of from about 20 to about 100 atmospheres.
In high temperature gasification, the process generally involves the reaction of carbon with air, oxygen, steam, carbon dioxide, or a mixture of these gases at 1300 F (700° C.) or higher to produce a gaseous product. Once a carbonaceous material is converted to a gaseous state, undesirable substances such as sulfur compounds and ash may be removed from the gas. The products of this process may include hydrocarbon gases (also called syngas), hydrocarbon liquids (oils) and processed feedstock/char (carbon black and ash), heat, and slag.
The by-products of high temperature gasification is slag, a non-leachable, non-hazardous a glass-like material which consists of the inorganic materials, which do not vaporize. In the high temperature conditions, the mineral matter melts and is removed as molten slag, which forms a glassy substance upon quenching or cooling. This material is suitable for use as construction materials. For example, the material may be crushed and incorporated into asphalt for use in roads and the like. Alternatively, the material may be utilized to replace cinder in cinder or building blocks, thereby minimizing absorption of water within the block. Further, the material may be solidified to a final form which is suitable for disposal without health risks or risks to the environment.
Chemistry of the Process
Gasification (the complete conversion of a carbonaceous feedstock to off-gas and then to syngas) can proceed at high temperature or low temperature, high pressure or low pressure and in one step or where the stages are separated to some degree under conditions (temperature, process additives) in a manner that certain reactions are favored over another. It can occur in one chamber, multiple regions within one chamber or multiple chambers. As the coal proceeds through a gasification reactor, physical, chemical, and thermal processes may occur sequentially or simultaneously, depending on the reactor design and the composition of the coal.
Drying occurs as the feedstock is heated and its temperature increases, water is the first constituent to evolve.
As the temperature of the dried feedstock increases, pyrolysis takes place. During pyrolysis the coal and biomass is thermally decomposed to release tars, phenols, and light volatile hydrocarbon gases while the coal is converted to char. Processed feedstock/char comprises the residual solids consisting of organic and inorganic materials. Depending on the origin of the feedstocks, the volatiles may include H2O, H2, N2, O2, CO2, CO, CH4, H2S, NH3, C2H6 and very low levels of unsaturated hydrocarbons such as acetylenes, olefins, aromatics and tars. Once a carbonaceous material is converted to a gaseous state, undesirable substances such as sulfur compounds and ash may be removed from the gas.
Gasification products are the result of chemical reactions between carbon in the processed feedstock/char and steam, CO2, and H2 in the vessel as well as the chemical reactions between the resulting gases. The gasification reaction is driven by heat (pyrolysis). This can be fueled by adding electricity or fossil fuels (e.g., propane) to heat the reaction chamber or adding air as a reactant to drive the exothermic gasification reaction, which provides heat to the reaction. Some gasification processes also use indirect heating, avoiding combustion of the coal in the gasification reactor and avoiding the dilution of the product gas with nitrogen and excess CO2.
This background information is provided for the purpose of making known information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.